This invention relates to a process for inhibiting scale formation in all types of dynamic water systems containing significant amounts of barium sulfate as well as other scale-forming salts commonly associated therewith, especially in downhole water systems. It also relates to a process for modifying the crystal structure of barium sulfate and to the crystals thus modified.
The inorganic salts deposited from dynamic water systems are commonly referred to as scale. The build-up of inorganic mineral scale deposits causes many problems such as loss of efficiency and accelerated corrosion in boilers and cooling towers and, in addition, in the latter acts as a site for the accumulation of organic foulants. In producing gas, oil, and geothermal wells, scale deposits in the flow lines and auxiliary equipment such as pumps, heat exchangers, and cooling towers can lead to expensive pipe replacement and downtime.
The types of scale encounters are to some degree similar whether in a boiler, cooling tower, or downhole system. They typically include calcium carbonate, calcium sulfate, calcium phosphate, iron oxide and, in some cases, barium sulfate and other heavy metal sulfate deposits.
Scale formation in downhole systems can result from the co-mingling of two fluid streams, each of which contains a concentration of a particular ion such that, when they commingle, an unstable water is produced. For example, in sulfate scale formation, one stream contains sulfate ions, and the other barium or calcium ions in concentrations such that an unstable water is produced. The mixing of these streams at the well bore results in the precipitation of hard crystalline barium sulfate and calcium sulfate deposits, which gradually build up on the walls of the well tubing to a point where they can choke off fluid flow in the tube if remedial measures are not undertaken. Another cause of scale formation in downhole systems is the precipitation of scale materials from supersaturated solutions containing the salts. When such solutions pass from the formation strata (where the temperatures and pressures are relatively high) into relatively low temperature and pressure areas, at or about the well base, precipitation of the salt on the tubing and surrounding strata occurs.
Barium sulfate scale is also a problem in papermaking. Although barium is present in low concentration, from sources such as wood, water, and additives, precipitation of the barium with the sulfate ion can cause scale and corrosion problems that are difficult to control. These deposits, which form mainly on screens, fan pumps, organ tubes, headbox and rectifier rolls, and the like, often cause formation problems and paper machine breaks. They also provide sites for anaerobic sulfate-reducing bacteria to develop, resulting in chemical and biological corrosion.
One of the ways scale build-up can be prevented is by reducing the concentration of ions in the water through ion exchange. In many cases, this is not economical and in others, such as downhole systems, it is impractical. Because of the costs and difficulty of treating the water systems externally, the addition of scale inhibitors has become increasingly popular. It is generally believed that scale inhibitors function by one or more of the following mechanisms: by dispersing the scale-forming ions, by enhancing the solubility of the scale-forming salts, and/or by modifying the crystal structure of the salts formed. Dispersants are thought to function by surrounding particulate materials with a barrier preventing random contact with other particles by either ionic repulsion or steric hindrance; they thus keep the salts in a loose flowable form. Solubility enhancers such as chelants and threshold inhibitors, i.e., substances which are able to control the solubility of large numbers of ions at low concentrations, increase the salt's solubility, thereby raising the temperature or concentration at which precipitation occurs. Crystal modifiers, such as organic polymers or foreign substances, affect the growth of the crystal or change the shape of the crystal by altering the axes and/or angles of the crystal.
In the petroleum industry, calcium sulfate and barium sulfate scaling is still a problem. Because of the inert character of these scales they are difficult to remove by chemical means. U.S. Pat. Nos. 3,806,451 and 3,962,110 (issued Apr. 23, 1974 and June 8, 1976 to Jack F. Tate) discuss the problem of removal including the use of strong alkali solutions. Treatment of calcium sulfate scale for 24-72 hours with concentrated potassium hydroxide and removal of the fluffy precipitate of calcium hydroxide thus formed is obviously undesirable or impossible. Moreover, alkali is not effective in preventing the build-up of scale deposits in the well tubing and production equipment.
The prior art discloses numerous processes for controlling the formation and/or deposition of scale using polymers. Of these, many disclose the use of acrylic and methacrylic acid polymers alone or in combination with other scale and/or corrosion control agents.
The following patents disclose the use of copolymers of acrylic or methacrylic acid with sodium vinyl sulfonate for scale control. They are directed to typical scales such as calcium sulfate, calcium carbonate, and/or calcium phosphate, magnesium silicate and iron oxide, as well as sludge and other foulants.
U.S. Pat. No. 3,682,224 (issued Aug. 8, 1972 to M. Bleyle) discloses the use of alkali metal or basic nitrogenous compounds of methacrylic acid-vinyl sulfonate copolymers (50-50 to 25-75%) for preventing the formation of carbonate and magnesium scale deposits in saline water evaporators.
U.S. Pat. No. 3,879,288 (issued Apr. 22, 1975 to F. H. Siegele) discloses the use of an aliphatic water-soluble copolymer of a monovinyl compound [e.g., (meth)acrylic acid, (meth)acrylamide, methacrylate, (meth)acrylonitrile, propylene, isobutene, 2-carbomethoxy propenoic acid, furmaric acid, and maleic acid] and about 25-75% mole % of a vinyl sulfonate including the acids, alkali salts, and allyl sulfonic acid for the control of hard, adherent scales, such as calcium and other alkaline earth metal salts, particularly carbonates and sulfates and/or iron. The copolymer should have a molecular weight of about 1000-25,000.
U.S. Pat. No. 4,361,492 (issued Nov. 30, 1982 to L. Dubin) discloses the use of acrylic acid-vinyl sulfonate copolymers as dispersants for particulate feric oxide. The copolymers generally contain 5-25 mole % of the vinyl sulfonate or its alkali metal salts (preferably sodium) and 95-75 mole % acrylic acid or its water-soluble alkali metal or ammonium salts. The molecular weights range from 500 to 50,000, preferably 1000-6000.
G.B. 2,110,659A (published June 22, 1983 by W. F. Lorenc et al.) discloses that the preferred water-soluble dispersing polymer is a vinyl sulfonate copolymer synthesized from 5-25 mole %, preferably 10-20%, vinyl sulfonate or its alkali metal (preferably Na) salts and from 95-75 mole %, preferably 90-80%, of acrylic acid and its water-soluble alkali metal or ammonium salts. The molecular weights range from 500-50,000, preferably 900-15,000 and most preferably 1000-6000. The copolymers are useful for calcium and magnesium scale forming salts.
Can. 1,148,438 (issued June 21, 1983 to M. Slovinsky et al.) discloses the use of specific copolymers of acrylic acid and/or its alkali metal or ammonium salts with an alkali metal vinyl sulfonate for reducing the scale on heat transfer surfaces. The copolymer has a mole ratio of acrylic acid to sodium vinyl sulfonate of from 90:10 to 60:40 and a molecular weight of 1,000-60,000.
Only five patents, U.S. Pat. No. 3,806,451 and 3,962,110 to A. Tate (cited previously), U.S. Pat. No. 4,008,164 (issued Feb. 15, 1977 to J. D. Watson), U.S. Pat. No. 4,530,766 (issued July 23, 1985 to W. M. Hann et al.), and U.S. Pat. No. 4,590,996 (issued May 27, 1986 to D. H. Hoskin et al.) even discuss the control of barium sulfate scale. Of these the Tate patents and Watson patent never exemplify its control, exemplifying only the control of calcium sulfate scale. The Tate processes incorporate in the aqueous system a tetrapropyl-tetraphosphoric acid and/or the alkali metal salts thereof (U.S. Pat. No. 3,806,451) or a water-soluble polyvinylpyrrolidinone with the system being made by the addition of up to 30 wt. % of a non-oxidizing mineral (U.S. Pat. No. 3,962,110). The Watson process involves the addition of a copolymer of acrylic acid and methyl acrylate (4-5:1 moles) having a molecular weight of 6000-8000. The Hann patent involves the addition of an acrylic acid - methacrylic acid copolymer (30-70% by weight of methacrylic acid) having a molecular weight of about 2000-5000. Scale control is exemplified in a mixed system where the barium ion is a minor component (136 out of a total of 20,279 mg./1). The Hoskin patent involves the addition of a polyalkoxy sulfonate. Scale control is exemplified in salt solutions (0.5M and 1.0M NaCl) containing barium sulfate (1.times.10.sup.-3 M and 0.5.times.10.sup.-3 M). In most cases, precipitation was only delayed from several hours to 24 hours. With several inhibitors there was no precipitation after several days. As noted below, barium sulfate is considerably more soluble in the presence of salt and it is not clear how effective this treatment would be in the absence of salt.
As pointed out by Hoskin, barium and strontium sulfate scales are of particular concern because of their extremely low solubilities (10.sup.-4 to 10.sup.-5 [Ba.sup.++ ] depending upon brine concentrations and temperature). At room temperature the solubility of BaSO.sub.4 in distilled water is about 2 ppm and at 80.degree. C. it is only about 4 ppm. In 0.5M NaCl, the solubility is 7 ppm at room temperature and about 30 ppm at 80.degree. C.; in 1.0M NaCl, the solubility is about 23 and 42 ppm, respectively. Brackish waters and dilute brines can contain up to 36 ppm Ba.sup.++, while other hard scale brines formed in mixing tanks and surface lines can contain from 75 to 180 ppm Ba.sup.++. The extreme insolubility of barium sulfate makes it very likely that scaling will occur if both barium and sulfate ions are present in a system. There is, therefore, a need for a process for effectively inhibiting the formation of barium sulfate scale and other heavy metal scales associated therewith, as well as calcium sulfate, calcium phosphate, and calcium carbonate scale typically also present in downhole systems.